The overarching goals of the NICHD Fetal Growth Studies are to 1) determine optimal growth standards for 4 racial/ethnic groups of fetuses and 2) to identify factors associated with decrements in expected fetal growth. Related goals are in developing algorithms for better estimating fetal weight and in determining the optimal timing of delivery. A major accomplishment was in the development of a database for the storage of 2D and 3D ultrasonology images and accompanying biometric data. This database is a valuable resource for follow on work aimed at determining specific growth dimensions such as bone length, volumes or organ size. This study will determine if race-ethnic standards are needed for assessing the appropriateness of fetal growth, established growth velocities for singleton fetuses, establish trajectories for fetal growth among twin pregnancies, and establish standards for fetal growth for obese pregnant women among other research questions.